1. Field of the Invention
The present invention relates to a structure of a photodiode image sensor device and a method for fabricating the same, and more particularly to a structure of a complimentary metal-oxide semiconductor (CMOS) image sensor and a method of fabricating the same.
2. Description of the Related Art
A photodiode image sensor is a commonly found image sensor element. A typical photodiode image sensor is a sensing region and comprises at least a reset transistor and a diode. For example, a diode is formed from a N-type doping region while a P-type substrate serves as the sensing region. In operation, a voltage is applied to the gate electrode of reset transistor so that after initiation of the reset transistor, the N/P diode junction capacitor is charged. When a high potential is attained during charging, the reset transistor is switched off, so that the N/P diode produces reverse bias to form a void region. When light strikes the N/P diode sensing region, the produced electron-holes are divided by the electric field of the void region such that electrons move toward the N-type doping region and the potential of the N-type doping region is reduced, and the electron-holes move to the P-type substrate.
A charge-coupled device (CCD) possesses dynamic range, low dark current and the technology development thereof is mature. As a result, it is the most commonly employed image sensor. However, the fabrication process of CCD is specific, which causes the costs to be expensive, and drive circuits must be operated with high voltage which causes high power dissipation. Further random access problems cannot be solved.
The CMOS image sensor possesses high quantum efficiency, low Read noise, high Dynamic range and random access and is fully compatible with the CMOS fabrication process. Thus, a CMOS can be integrated onto a chip with other controlled circuits, A/D converter circuits and digital signal treatment circuits, thereby creating a system on a chip (SOC). Thus, a CMOS image sensor fabrication process greatly reduces the cost of an image sensor, pixel size, and power dissipation. Accordingly, in recent years, CMOS image sensors have been used to replace CCDs.
A conventional fabrication method of CMOS image sensor is briefly described as follows:
Referring to FIG. 1A, first, a field oxide layer 102, a gate oxide layer 104 of a reset transistor 130 and a polycrystalline silicon gate electrode 106 are formed on a substrate 100. Next, the field oxide layer 102 and the polycrystalline silicon gate electrode 106 are used as an implant masking screen for an ion implantation and thermal driving in fabrication process whereby a source/drain region 108 and a doping region 112 of the photodiode sensing region 110 are formed in the substrate 100.
After that, at the lateral wall of the polycrystalline silicon gate electrode 106 and the gate electrode oxide layer 104, a spacer 114 is formed. After that, a self-aligned barrier (SAB 116) is formed on the photodiode sensing region 110 so as to form a photodiode CMOS image sensing element.
The backend process of forming a photodiode CMOS image sensing element is shown in FIG. 1B. In the backend process, for example, a dielectric layer 118 is formed on the substrate 100, and then a metallic conductive wire 120 is formed is formed on the dielectric layer 118. After that, a dielectric layer 122 is formed over the substrate 100, and a metallic conductive wire 124 is formed on the dielectric layer 122.
However, the CMOS image sensor obtained by the conventional fabrication process has the following drawbacks:
In the course of the backend process, for instance, in defining the contact window/dielectric layer window opening and defining the metallic conductive, a plasma etching process must be used. This plasma etching method possesses relatively large energy which will cause a relatively large voltage drop. This voltage drop not only damages common transistor elements but also damages the surface of the photodiode sensing region. In particular, the damage by the plasma at the bird""s beak region of the field oxide periphery is more serious. Thus, the photodiode sensing region produces current leakage. This current leakage problem in turn causes the sensor to produce a relatively large dark current, causing Read Noise to increase.
Further, in the course of forming a plurality of sensors on the same wafer, the numerical size distribution of the dark current for a single sensor has a relatively large range, ie., the difference between the numerical values of the dark current of every sensor can be very large and this causes non-uniformity in the sensors.
Accordingly, it is an object of the present invention to provide a structure of a CMOS image sensor and method for fabricating the same, wherein a protective layer is formed between the surface of the element and the dielectric layer, and is also formed between each dielectric layers to prevent damage by plasma.
Yet another object of the present invention to provide structure of a CMOS image sensor and method for fabricating the same, wherein the problem of dark current can be reduced to a minimum.
Another object of the present invention to provide structure of a CMOS image sensor and method for fabricating the same, wherein the uniformity of a CMOS image sensor can be increased.
One aspect of the present invention is to provide a structure of a CMOS image sensor comprising a photodiode sensing region, a transistor element region, a transistor, a self-aligned barrier, a dielectric layer and a first, second protective layers, wherein the photodiode sensing region and the transistor element region are located on the substrate and the transistor is located on the transistor element region. The self-aligned barrier is located on the photodiode sensing region, and the first protective layer is located on the entire substrate and covers the self-aligned barrier. A plurality of dielectric layers are each formed on the first protective layer, wherein each of the dielectric layers is separated from the first protective layer by different heights, and a plurality of second protective layers, each of which is mounted on the adjacent dielectric layers.
Yet another aspect of the present invention is to provide a method of fabricating a CMOS image sensor. An isolation layer is formed on a substrate to partition the substrate into a photodiode sensing region and a transistor element region. Next, on the transistor element region, a gate electrode structure is formed and a lightly ion implantation process is performed such that a lightly drain doping region is formed on the transistor element and a lightly doped region is formed on the photodiode sensing region. After that, a spacer is formed on the gate electrode structure and a high ion implantation step is performed so that a source/drain region is formed on the transistor element and a highly doped region is formed on the photodiode sensing region. After that, after a layer of self-aligned barrier is formed on the photodiode sensing region, a layer of protective layer is formed on the substrate so as to cover the entire substrate. The material between the protective layer and the self-aligned barrier layer possesses different refractivity. After that, the backend process of the sensor element is performed. In the backend process, by forming successive dielectric layers and metallic conductive wires on the protective layerand then a protective layer thereon, in which the number of dielectric layers and the metallic wires depend on the fabrication process, a protective layer is formed between each dielectric layers.
In view of the above, the present invention is characterized by, after the formation of the photodiode CMOS image sensor, covering the entire substrate with a protective layer. The invention is further characterized by forming a protective layer between dielectric layers in the back-end process. By means of the action of the protective layers, damage to the photodiode sensing region due to the application of the plasma etching used in the back-end process can be avoided, and in turn, the production of the dark current is reduced to a minimum.
Further, other than the essential protected photodiode sensing region, these protective layers cover the entire substrate, and other remaining regions are also protected from the damage of the plasma etching fabrication process.
In addition, with respect to a plurality of sensors on the same chip, the sensor having a plurality of protective layers is provided with dark current having its size distribution concentrated within a dense range. Thus, the dark current characteristics of those sensors are near to each other, i.e., these CMOS image sensors are provided with relatively excellent uniformity.
In addition, the protective layer and the material of the self-aligned barrier and the dielectric layer have different refractivities. Due to refraction of an incident ray entering the surface of the photodiode sensing region by the protective layer and the dielectric layer, the self-aligned barrier of material having different refractivities, and the photodiode sensing region absorbing the refracted incident ray, the photodiode region is provided with a preferred conversion ability to generate photons and thus the photo diode sensing region has a higher quantum efficiency.